Field of the Invention
The present invention relates generally to the field of fiber optics, and in particular to an improved time domain multiplexing (TDM) and wavelength division multiplexing (WDM) based FBG sensor array system.
Background Art
A typical fiber optic sensor may comprise an array of sensing elements, located at respective sensing points, such that each generates a respective optical output that varies in response to changes in temperature and/or strain. One popular type of fiber optic sensor uses fiber Bragg gratings (FBGs) as sensing elements. An FBG is an in-fiber optical device that reflects light at a known wavelength (i.e., the “Bragg wavelength”) that shifts in response to changes in temperature and/or strain. In an FBG sensor system, a broadband input laser beam is coupled into an array of FBGs. The output of the sensor array (i.e., the light reflected back from the FBGs) is then coupled into an output processing stage, which uses the sensor output to derive precise measurement data.
One significant advantage of using FBGs as sensing elements is that they lend themselves to multiplexing. Outputs from all of the FBGs in a sensor array can be transmitted to the output processing stage over a single optical fiber pathway. The output processing stage then reconstructs the output of each individual FBG in the sensor array. The wavelength change for each grating is then used to calculate the physical parameter(s) to be measured, such as temperature and/or strain. The output data can be multiplexed in different ways.
A wavelength division multiplexing (WDM) can be used to multiplex an optical signal in the wavelength domain. In a WDM-based sensor system, the FBG sensor array is configured such that each individual FBG provides an output within a unique, non-overlapping wavelength range. Thus, the respective contribution of each FBG to the multiplexed signal is identifiable by wavelength. WDM-based designs are widely used for a number of reasons, including the fact that the technology is well established, straightforward and easy to implement.
Alternatively, a time domain multiplexing (TDM) technique can be used to multiplex an optical signal in the time domain. In a TDM-based sensor system, the input into the sensor array comprises a series of discrete pulses. The individual FBGs within the array are spaced apart, such that the response of each successive FBG to an input pulse is delayed by a duration in time greater than the duration of the input pulse. Thus, when an input pulse propagates through the sensor array, an output signal is generated that comprises a series of pulses, each generated by the partial reflection of the input pulse by a respective FBG. The output processing stage can then use the arrival time of an output pulse to determine which FBG in the sensor array was the source of that particular pulse.
Conventional multiplexing techniques are proving to be inadequate for today's most demanding applications. In some fields, such as civil engineering, applications are calling for increasing numbers of sensing points over greater and greater distances. For example, a civil engineering application may call for thousands of sensing points over a 10 km span.
A conventional WDM-based sensor system is incapable of providing such a large number of sensing points. As discussed above, a WDM-based sensor requires each individual FBG in the sensor array to operate within a unique, non-overlapping wavelength range. Thus, an application calling for thousands of sensing points would require a wavelength range exceeding the capacity of current systems.
Conventional TDM-based systems are also problematic, but for different reasons. As mentioned above, in a TDM-based system, individual FBG outputs are identified by their arrival time rather than by their wavelength. Thus, wavelength range is not an issue. However, in a sensor system with thousands of sensor gratings, the output processing stage has to be able to robustly and accurately reconstruct the short pulse and wavelength response of each FBG in the sensor array from a received sensor output having a frequency on the order of 108 Hz or faster. Thus, it is desired to have a sensor and/or a sensor system that can meet such demands.